Glycinebetaine (GB) is a compatible organic solute produced by several plant species to cope with salt or water stress. This compound is involved in osmotic adjustment as well as in protecting several cellular structures. It is well established that rice is unable to synthesize this compound because it lacks choline monooxygenase(CMO), which is involved in the synthesis of betaine aldehyde, the immediate precursor of GB. It has also been reported that exogenous GB protects rice from salt stress by maintaining photosystem II integrity and relative water content (Harinasut et al 1996). Its effect on sodium nutrition in salt-stressed rice, however, was never reported.
In a phytotron experiment, a salt-sensitive rice cultivar--I Kong Pao--maintained on nutrient solution (Yoshida et al 1976; renewed each week) was subjected to various NaCl doses (0, 30, 50, and 100 mM), with or without simultaneous exposure to 1 mM GB (Sigma Chemical). Plants were exposed to salt stress at the seedling stage (20 d old; 100 plants per treatment) on 25-L tanks in a complete randomized block. Ten surviving plants were collected each week during a subsequent 1-mo experiment. Relative growth rate was quantified on a dry-weight basis and ion concentration was determined after digestion of organic matter with nitric acid by an inductively coupled argon plasma emission spectrophotometer.